Hemiparesis refers to motor weakness involving the arm and leg on the same side of the body. Hemiparesis may occur with a lesion in the brain, brainstem, cerebellum, or upper cervical spine (Figure 237.1 A-D). Hemiplegia, whether spastic or flaccid, always implies a central nervous system lesion. Hemiplegia does not occur as a consequence of peripheral nervous system involvement.
Unilateral lesions in the cortex, centrum semiovale, internal capsule, midbrain, and upper pons may produce contralateral hemiparesis and facial weakness (Figure 237.1 A).
Unilateral cerebellar lesions may produce hemiparesis on the same side of the cerebellar lesion. The face is usually spared.
Unilateral pontine lesions produce contralateral hemiparesis and ipsilateral facial weakness (Figure 237.1 B). The involvement of the face on the same side as the lesion (on the side opposite from the hemiparesis) is due to direct damage to the facial motor nucleus or its fibers. The sparing of the face on the side of the hemiparesis occurs because the fibers innervating the facial musculature of the opposite side cross to the other side at the level of the midbrain. The presence of contralateral hemiparesis and ipsilateral facial weakness is referred to as cross-hemiplegia.
Unilateral upper medullary lesions may produce contralateral hemiplegia with sparing of the facial musculature (Figure 237.1 C). Medullary lesions more commonly produce paraparesis. They damage the pyramidal system as their left and right fibers cross to the other side.
A unilateral upper spinal cord lesion may produce ipsilateral hemiplegia with sparing of the facial musculature (Figure 237.1 D).

Figure 237.1.— Possible sites of anatomical injury producing hemiparesis. A: brain and midbrain; B: upper pons; C: lower pons and medulla; D: rostral spinal cervical cord; V: ventricles; T: thalamus; FN: facial nerve; UQ: upper quadrant; LQ: lower quadrant; BP: brachial plexus; LSP: lumbosacral plexus. The colored rectangles indicate the location of weakness produced by damage to the various components of the somatic motor system.